SQUID-Detected Magnetic Resonance Imaging in Microtesla Fields

The use of very low noise magnetometers based on Superconducting QUantum Interference Devices (SQUIDs) enables nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) in microtesla magnetic fields. An untuned superconducting flux transformer coupled to a SQUID achieves a magnetic field noise of 10⁻¹⁵ T Hz^−1/2. The frequency-independent response of this magnetometer combined with prepolarization of the nuclear spins yields an NMR signal that is independent of the Larmor frequency ω0. An MRI system operating in a field of 132 μT, corresponding to a proton frequency of 5.6 kHz, achieves an in-plane resolution of 0.7 × 0.7 mm² in phantoms. Measurements of the longitudinal relaxation time T1 in different concentrations of agarose gel over five decades of frequency reveal much greater T1-differentiation at fields below a few millitesla. Microtesla MRI has the potential to image tumors with substantially greater T1-weighted contrast than is achievable in high fields in the absence of a contrast agent.